Scaling of device dimensions has been a primary factor driving improvements in integrated circuit performance and reduction in integrated circuit cost. Due to limitations associated with gate-oxide thicknesses and source/drain (S/D) junction depths, scaling of existing bulk MOSFET devices below the 0.1 μm process generation may be difficult, if not impossible. New device structures and new materials, thus, are likely to be needed to improve MOSFET performance.
One factor that limits the size of MOSFET devices is the size of the contacts. For example, transistors typically have several contact holes associated with the source, drain, and gate. The size of the contact holes partially determines the size of the transistors. This also holds true for Static Random Access Memory (SRAM) cells and flash memory cells. In fact, the contact size affects the cell size of almost every semiconductor technology. By shrinking the contacts, the size of the MOSFET devices may be reduced.
Another factor that affects the size of MOSFET devices is contact pitch (i.e., the spacing between contacts). The contact pitch is usually limited by lithography.
Existing processes for forming contacts include lithography techniques, where contact openings are printed in a photoresist, followed by an etch process to transfer the opening to the contact level. It is usually difficult to print small contacts by lithography (i.e., using small holes in the reticle and low exposure energy) since this leads to a small and usually unmanufacturable lithography process window in terms of exposure and process latitude. It is, therefore, advantageous to be able to print the actual contact hole at a larger size lithographically, but use an appropriate shrink technique in order to reduce the dimension of the eventual contact. Other techniques include a spacer-based contact shrinking technique and a Resolution Enhancement of Lithography by Assist of Chemical Shrinkage (RELACS) technique. The formation of small contacts (e.g., less than 100–140 nm), however, is difficult to achieve using existing techniques.